1. Field of the Invention
The present invention relates to an asymmetric digital subscriber line (ADSL) system, and in particular, to a method for controlling transmission power in an asymmetric digital subscriber line system.
2. Description of the Related Art
In general, an asymmetric digital subscriber line (hereinafter referred to as an ADSL) system transmits a signal at a predetermined fixed transmission power level, when transmitting data through a telephone line interconnected between an ADSL-TUC (Transmission Unit Center) and an ADSL-TUR (Transmission Unit Remote). Preferably, the transmission power level should be sufficiently high so as to maintain the preferred data transfer rate in consideration of an S/N (signal-to-noise) ratio due to attenuation of the signal. On the other hand, the transmission power level should be sufficiently low so as to reduce any influence due to crosstalk between the subscriber lines, wherein the crosstalk is proportional to the transmission power level.
Conventionally, in the light of the foregoing, the transmission power level in an ADSL system has been set at a fixed, appropriate power level. Unfortunately, this results in inflexibility in improving the transmission capability. In reality, the telephone lines of the respective subscribers may have different conditions of noise, attenuation, and crosstalk susceptibility. Nevertheless, the transmission power level has been fixedly set with respect to all the subscriber lines, so that in some cases, the fixed transmission power level may be lower or higher than an appropriate power transmission level which may be different depending on the subscriber lines. As a result, the system may be influenced by crosstalk. In order to reduce the negative affects of the crosstalk, the data transfer rate must be lowered. This results in a decrease of the transmission capability and an unnecessary waste of transmission power.